Single side band modulator



Dec. 26, 1950 R. R. YOST, JR 2,535,340

SINGLE SIDE BAND MODULATOR Filed Nov. 27, 1945 2 Sheets-Sheet 1 INVENTOR.

RUSSELL R. YOST JR. BY M 9,44%.

A TTOR/VEY Dec. 26, 1950' R. R. YOST, JR 2,535,340

SINGLE SIDE BAND MODULATOR Filed Nov. 27, 1945 2 Sheets-Sheet 2 FIG. 2 FIG?) 1 fc+fm a m INVENTOR.

RUSSELL R. YOST JR.

MwEM

ATTORNEY Patented Dec. 26, 1950 UNITED STATES PATENT OFFICE SINGLE SIDE BAND MODULATOR Russell R. Yost. Jr., Port Washington, N. Y., assignor to the United States of America as represented by the Secretary of War Application November 27, 1945, Serial No. 631,180

Claims. 1

This invention relates to electrical circuits and more particularly to a type of circuit known as a single-sideband modulator circuit.

It is often desirable to produce a signal that differs in frequency by a predetermined amount from a given signal. One use of a circuit that produces such a signal is in testing relative-velocity measuring devices or any radio devices which depend upon the Doppler effect for their operation. These devices are responsive to the changed frequency of a reflected received signal compared with the frequency of an emitted signal. Elaborate signal generators have in the past been used to test the above mentioned Doppler effect devices, but their construction involves driven mechanical components for developing the equivalent of the reflected waves. Such test generators are relatively inflexible in use since they can be designed for operation over only a restricted range of radio frequencies and of relative velocities. It has also been proposed to use an ordinary oscillator operating at a known frequency which differs by any necessary amount from that of the signal emitted by the device to be tested. The tendency of such an oscillator and its device to shift their frequencies towards coincidence has prevented successful application of this principle where those frequencies are even moderately close to each other.

A second application of a circuit for performing the above mentioned operation is in radio modulator circuits where it is desirable to obtain only a single sideband from the combination of a carrier frequency signal and a modulating signal. In the past the separation of one sideband from the second sideband and carrier frequency has been performed by the use of tuned circuits having very sharp frequency response characteristics. It is impossible, however, to provide tuned circuits with sufficiently sharp frequency response characteristics to separate two frequencies that are very close together, for example, a signal of 100 megacycles and a second signal of 100 mega-r cycles plus 20 cycles.

It is an object of the present invention, therefore, to provide a circuit for producing a signal that differs in frequency by a predetermined amount from a given signal.

It is a further object of this invention to provide a modulator circuit that is capable of providing a signal, the frequency of which is equal to the sum or the difference of two frequencies whichever may be desired.

A third object ofthis invention is to provide a modulator circuit in which the amplitude of the carrier frequency and at least one of the sidebands may be adjusted at any value including zero.

In accordance with the present invention there is provided a plurality of means for modulating a given signal with a second signal. Means are provided for shifting the phase of the two above mentioned signals and for amplifying the first mentioned signal by any desired amount. Means are also provided for combining the outputs of the modulating means and said amplifying means whereby a signal having a frequency equal to the sum or difierence of the frequencies of the two above mentioned signals is obtained.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing in which:

Fig, l is a wiring diagram of one embodiment of the present invention;

Figs. 2 through 8 are vector diagrams of signals present in the circuit of Fig. 1; and

Fig. 9 is a vector diagram of a more general case than that illustrated in Figs. 2 through 8.

In Fig. 1 a signal fc from any source (not shown) of a predetermined frequency is applied to the input transformer 19, the secondary of which is tuned by split capacitor [2. A modulation signal fm is obtained from a suitable source (not shown) and appied to the circuit through transformer l4. Half of the voltage across the secondary transformer I4 is applied to a potentiometer IS. A phase shifter comprising variable resistor 18 and capacitor 29 is connectedacross the entire secondary of transformer 14. A potentiometer 22 similar to potentiometer I6 is connected between the junction of resistor 18 with capacitor 29 and the center-tap of the trans former M. The tap on potentiometer 22 is mechanically coupled to the tap on potentiometer It in such a manner that moving the tap on potentiometer 22 to a point of greater signal amplitude moves the tap on potentiometer IE to a point of lesser signal amplitude.

The signal fc is modulated by si nal fm in two modulator tubes 24 and 25. A resistive-capacitive couping network comprising capacitors 28 and 30 and resistor 32 is provided for applying both signals jc and fm to the control grid of tube 24. Capacitors 28 and 38 have substantially zero impedance to the signal fc and substantially infinite impedance to the signal fm. The voltage at the frequency fc that is applied to the grid of tube 24 is developed across resistor 32 while the voltage of a frequ ncy fm is d veloped acro s capacitor 30. Capacitor 28 prevents the signal fm from appear ng at tran former l3 and capacitor 3 prevents the signal f from appearing at transformer M. The tap on potentiometer 22 is connected to the junction of resistor 32 and capacitor 30. A resistor 34 and a capacitor 36 so chosen that the impedances presented to signal fc by the two element are sub tantially equal are connected between gro nd and one t rminal 37 of the secondarv of tran former ID. The junction of res stor 34 and capacitor 36 is connected to the control grid of tube 24 through capacitor 28. A resi tive-capacitive network compri ing capacitors 38 and 49 and re istor 42 is provid d for coupling the signals fc and fm to the control grid of tube 26. The tap on potentiometer I6 is connected to the junction of re istor 42 and capacitor 40. The series combination of a capacitor 44 and a resistor 46 is connected between ground and terminal 48 of transformer I I Again capacitor 44 and resistor 45 are so chosen that their imp dances at the frequency of signal fc are substantially equal. The junction of re istor 45 and capacitor 44 is conn ct d through capacitor 38 to the control grid of tube 26.

A pha e shifter com rising resistor 50 and variable capacitor 52 is connected between terminals 37 and 48 of transformer Ill. The junction of resistor 50 and capacitor 52 is connected to the control grid of a vacuum tube 54. Vacuum tube 54 as well as vacuum tubes 24 and 23 are preferably of the pentode type and preferably have a hi h p ate im edance to prevent cross modulation of t e signa s from the three tubes. The cathodes of tubes 24. 2%, and 54 are all connected to ground, and in each tube the suppressor grid is directly connected to the cathode. A source of direct current potential illustrated schematical y by battery 55 w th re istors 58 and en connected between the positive and negative terminals of batt r 56 provide a fixed bias for the control grids of tubes 24 and 26. This bias is applied to these tubes by connecting the junction of resi tors 58 and 60 to the cent r tap of the secondary of transformer M. The positive terminal of batter is connected to ground r sistor s shunted by a capacitor 62 so that the iun tion of resi tor 58 and resistor 60 is at ground otenti l for both signals in and fm. A v riable bias is applied to the control grid of tube 54 by conn cting potentiometer 4 between the ositive and negative terminals of b ttery 58. A movab e ta on potentiometer 64 is connected to the center tan of the secondary of tra sformer m. The cent r tap of transformer I0 is connected to ground for all of the alternating current signals by a ca acitor 66. The anodes of tube 24, 2 and 54 are connect d together and to a source o positive otent al through a c il 68 and a resi tor I Coil 8 is tuned to re onate at the frequency of t e de ired output signal by a variabl ca citor 2. The unction of res stor and coil R8 is connect d to ground through a ca acitor 74. Res stor m and capacitor 74 form a type of conventional d coupling filter. tive otenti l supp y for the screen grids of tubes 24. 2% and 54 is provided b means of a volt ge divider 79 connected from 18+ to ground. The screen grid of each of the e three tubes is connected to t e center tan 8 of this potential. divid r through a conventional resistive-capacifive p g n twork 2. as shown in F g. l- The c nt r tap 8'! of voltage divid r 9 is maintained at ground potenti l for alt rnating current signals by means of a capacitor 84. A tap on coil 4 68 connected through a capacitor to an output terminal 88 provides means for obtaining a signal from this circuit.

The operation of the circuit of Fig. 1 may best be understood by reference to Figs. 2 through 8, which are vector diagrams of signals present in the circuit of Fig. 1.

Suppose now that a signal fc is applied to transformer 10 and a signal fm is applied to transformer l4. Suppose further that the vector fc of Fig. 2 represents the signal fc applied at the input of tube 24 and that vector fm making an angle (1: with a vertical reference line represents the signal fm applied to the input of tube 24. The signal at the anode of tube 24 may now be represented by the system of vectors shown in Fig. 3. At a particular instant of time the vector fc represents the amplitude and phase of the signal f0 and the two oppositely rotating vectors fc+fm and fcfm represent the upper and lower sidebands, respectively. These two vectors will be displaced by eoual angles c from vector fc, and the angular velocity of these two vectors with respect to the vector ,fc is proportion l to the frequency fm. Fig. 4 represents in vector form the signals fm and fa that are applied to the control grid of tube 25. Since the impedance of capacitor 3'5 was made equal to that of resistor 34 and the impedance of capacitor 44 was made equal to that of resistor 4-5, the vector f(:' of Fig. 4 will lag 90 degrees behind the vector is of Fig. 2. At the same time resistor 18 may be adjusted so that the vector fm, Fig. 4, lags 90 degrees behind the vector fm of Fig. 2. The vector fm' makes an angle with the vertical reference line. Fig. 5 is a vector diagram of the out ut of tube 25 at the same time as that represented in Fig. 3. The vector f0 represents the signal fc' and vectors fc+fm and fc-fm' represent the upper and lower sidebands. These two last mentioned vectors rot te in op osite directions with res ect to vector fc and at this instant make an angle of 5 with vector 1%.

Since the outputs of tubes 24 and 26 are added vectoriallv in the tuned circuit comprising coil 68 and capacitor I2, the vector diagrams of Fig. 3 and Fig. 5 m y be added to obtain the net output from tubes 24 and 26. Fig. 6 is a series of vector diagram-: repre entin this addition. Since vectors fc+fm and fc'+fm' represent signals of the same am litude and freouency but are in phase opposition the sum of these two sign ls will be z ro. If the e two si nals are not e u l in amplitude, the may he m de so b adu ting potentiometers l 8 and 22. In a sim lar manner vectors fefm and c'-fm' re resent two signals of the sam fre uencv and am itude and in phase with each other. The sum of these two vectors is a vector r tating at a fr ouen y eoual to that of the vectors ffm and fc'fm' but twice the ampl tude of eit er. The vector in" re resents the vector sum of vectors ,fc and fa. Fig. 7 repre sents the ad ition of a vector fc' to the resultant vector of Fig. 6. The vector fc represents the amplitude and hase of a signal eou l in frequency to signal fc at the out ut of tube 54. The amplit de of fc is ad usted by moving the tap on potentiometer 64 which changes the bias on tube 54, and the pha e of signal fc is adiusteci by me ns of capacitor 2. The out ut of tube 54 is added vectoria lv to the out uts of tubes 24 and 26 n th tun d cir u t containing coil 68. Therefore. if the ignal fc'" is made eoual in ms gnitude but in phase opposition to signal fc" the net signal across coil 68 will be that signal represented by the vector (fc-fm) +(fcfm) Fig. 8.

The vector of Fig. 8 represents a signal of a frequency equal to the difference in frequency between signals fc and im. If signal fc has a frequency of 100,000,000 cycles per second and signal fm has a frequency of 100 cycles per second, the signal appearing across coil 68 and hence at terminal 88 will be a signal of 99,999,900 cycles. The percentage difference in frequency between the two signals or either signal and either sideband makes no difference in the operation of this circuit because no tun d circuits are employed to separate signals of different frequencies.

If it is desired to obtain the upper sideband or, in this case, a signal of 100,000,100 cycles per sec and, the position of capacitor 20 and resistor 18 may be interchanged so that the signal fm leads signal fm by 90 degrees. If potentiometers I6 and 22 are oppositely ganged as stated above, complete or incomplete cancellation of the unwanted side may be obtained as desired. In a similar manner complete or incomplete cancellation of the si nal fc may be obtained by adjusting capacitor 52 and potentiometer 64.

In Fig. 9 a broader aspect of the invention is illustrated. The analysis to this point has assumed that perfectly equal and symmetrical sidebands are attainable. Also, the carrier of the two modulated signals have been considered to be at a 90 angle to each other. The former condition is not perfectly attainable in practice. Neither condition is essential to theoretically perfect results. Vector F represents for example the received signal voltage from a device to be tested. Vectors F1 and F2 represent sidebands of vector F, unequal in magnitude and unsymmetrical to vector F as they may be in practice. A carrier F the frequency of which equals that of vector F but which is displaced therefrom by other than 90 is modulated by a signal frequency equal to the difference between vector F and its sid bands. Before the two modulated signals are combined, the unwanted sidebands F1 and F1 must be made equal in magnitude and opposite in phase. Even if the sidebands F2 and F2 of the desir d frequency are then not in phas they will in all probability yield a desired resulta t F5. The resultant F", of carrier frequency, may readily b eliminated by an equal and op osite voltage F'.

Regardless of the specific equipment designed for acco plishing t e proposed method of signal gen ration. that method yi lds a signal which is stable in frequency relative to the giv n si nal, and which avoids the tendency charact ristic of simple oscillators to lock in with associated equipment op ratin at a n arly q al frequ c By providing an entirely el ctrical unit, the shortcomin'rs and the limitations as to operating frequ ncy range a d range of frequency difference. limitations c aracteri tic of alternative signal generators involving driven parts, are avoided.

The system described may be constructed using lum ed. adjustable compon nts l0, I2. 34, 36. 44. 46, 68, 12, 50, and 52 for frequencies up to a few hundred megacycles and may be adapted with known U. H. F. components to higher frequencies. The RC phase-shifting devices 34. 36, 44, 45 50 and 52 are here shown as ener iaed from a single coil, but it is evident that multipe coil s c ndari s mav alternativ ly be used. one for each pha e s ifter. The circuit shown is both simpl r a d more dependable. Also, with sacrifice of simplicity tube 54 may be omitted and tubes 24 and 26 replaced by two balanced modulator circuits to suppress the carrier frequency in each channel.

In this embodiment of the invention the desired results are attained by first mutually shifting the modulation signak and the carriers for the two channels and then modulating the carrier. The same result may be obtained if the modulation signal components are mutually shifted before modulation as shown, and the circuit is modified so that the entire modulated signals are mutually shifted in phase before they are combined. In that event no phase shifters (34, 35 and 44, 45) may be required for the components of the carrier signal supplied to the two channels. Phase shifters for the modulated sig- 118.15 would be required in their place.

As a further feature of the invention, where a given signal has multiple, reasonably close frequency components and it is impressed on the circuit in Fig. l, the respective frequency compone-tits will all be shifted by the substantially same amount, with appropriate circuit design. Where the multiple frequency components deviate materially from that for which the cancellation is designed, the cancellation of some of the original frequency components in the output will be imperfect but effective.

The embodiment of the invention shown may be used to test certain radio altimeters. These altimet rs emit a signal whose frequency is modulated to rise and fall at a constant rate, periodically reversing. The reflected wave also rises and falls in frequency, with a time lag. At any instant there is a frequency difference between the emitted and refl cted waves that represents altitude. The sign of the difference is successive- 1y positive and negative. Since the altimeter is re ponsive alike to equal frequency di f rences of opposite sign, a sustained frequency difference of one sign as provided by the generator will simulate the refl cted wave, to cause substantially the same altimeter reading as a reflected wave of equal frequency difference.

While there has been described what is at present considered the preferred embodiment of the invertion, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention.

What is claimed is:

1. A single sideband modulator circuit compri ing a first tuned transformer for introducing a fir t signal, a first and a second phase shifter connected to a first and a second half of the outp t of said transformer for providing a second and a third signal in phase quadrature, a third pha e shifter connected to the entire output of aid tuned tr nsformer for providing a fourth signal mutually shifted in p a e with respect to the t ree above mentioned signals, a second transformer means for introducing a fifth signal, a fourth pha e shifter connected to the output of said second tran former for providing a sixth signal mutually shifted in phase with re pect to said fifth signal, first and second potentiometer means for adj sting the amplitudes respectively. of said fifth and said sixth signals. a first vacuum tube modulator circuit providing means for modulating said econd signal with said fifth signal, a second acuum tu e modul tor circuit providing mean or modulating said third signal with said sivt signal, a vacuum t be amplifier circuit providing means for amplifying said fourth signal,

means for controlling the gain of said amplifier circuit and means for combining the outputs of said first modulator circuit, said second modulator circuit and said amplifier circuit in a common impedance whereby the amplitude of a seventh signal having the same frequency as said first signal, an eighth signal having a frequency equal to the sum of the frequencies of said first signal and said fifth signal and a ninth signal having a frequency equal to the difference of the frequencies of said first signal and said fifth signal may be selectively and individually varied in amplitude through a range of values including zero.

2. Apparatus for developing a signal having a desired frequency-difference relative to a signal of .given frequency, comprising means producing mutually phase-shifted components of a modulation signal the frequency of which equals said frequency difference, means including three channels for producing said signal of given frequency in said three channels phase shift means in at least two of said channels for separately shifting the phase of said signal of given frequency, means in said two of said three channels for modulating said given frequency with a respective component of said modulation signal means combining the modulated signals of said two channels in such phase and proportion that the unwanted sidebands are cancelled. and means combining the modulated signals of said two channels and the signal of given frequency of said third channel in-such phase and proportion that the third channel signal of given frequency cancels the re ultant of the given-frequency components of said modulated signals.

3 The method of developing a signal a desired frequency-difference relative to a signal of given frequency, which comprises the steps of providing a pair of mutually phase-shifted components of a modulation signal the frequency of which equals said frequency-difference, separately modulating each of a pair of given-signal components with a respective one of said modulation'components, combining the modulated signals in such phase and proportion that the unwanted sidebands are mutually substantially cancelling, and eliminating the carrier components of the modulated signals by combining them with a third component of said given signal which is of equal amplitude and opposite phase to the resultant of said carrier components, the resultant of the remaining sideband components being the de ired signal.

4. The method of developing a signal having a de ired frequency difference relative to a signal of given frequency which comprises the steps of generating a pair of mutually phase shifted components of a modulation signal, the frequency 60 of which equals said frequency difference, said mutual phase shift being any angle other than degrees or multiples thereof, generating a pair of mutually phase shifted components of said given frequency signal, the mutual phase shift of said given frequency signal components equalling 180 degrees minus the mutual phase shift of said modulation signal frequency components, modulating each given frequency signal component with a respective one of said modulation frequency components, combining the modulated signals with such relative amplitudes thereof that one of the side band frequencies of the modulated signals is cancelled, and eliminating the given frequency signal components of the modulated signals by combining them with a third component of said given frequency signal which is of equal amplitude and opposite phase to the resultant of said given frequency signal components.

5. A single side band modulation circuit com prising means providing a carrier frequency signal in three channels, means providing a modulating frequency signal in the first and second of said channels, first modulator means in the first of said channels for modulating said carrier frequency signal with said modulation frequency signal to produce sideband frequency and carrier frequency components, phase shift means in the second of said channels for separately shift ing the pha es of said modulation frequency signal and said carrier frequency signal so that the sum of the phase shift equals 180 degrees, sec ond modulator means connected to said phase shift means for modulating said phase shifted carrier frequency signal with said phase shifted modulation frequency signal to produce sidebanci. frequency and carrier frequency components, mixer means connected to receive the outputs of said first and second modulator means, means for adjusting the relative amplitudes of said outputs so that one sideband frequency component cancelledv and means in the third of said channels connected to said mixer for shifting the phase and adjusting the amplitude of said carrier frequency signal in said third channel so that it will cancel the resultant of the carrier frequency components of the outputs of said first and second modulators.

RUSSELL R. YOST, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,744,044 Green Jan. 21, 1930 2,163,719 Usselman June 27, 1939 2,173,145 Wirkler Sept. 19, 1939 

